Reinforcing steel comes in bars which are normally tied together to form a cage. The bars come in different grades or strengths and can be smooth or deformed (with small ridges). Sometimes the reinforcing is in a ready-made mesh which comes in standard sized sheets or rolls.
Reinforcing gives concrete structures their strength and also prevents shrinkage cracking. Engineers design the reinforcing that goes in the structure. It’s important that the reinforcing is placed in the positions indicated on the reinforcing drawings, and that where the reinforcing (or mesh) joins it overlaps with the next bars. It’s obvious that the right size bars (or mesh), of the right grade and deformations is used, otherwise the structure could fail.
Reinforced concrete is usually used in foundations and concrete slabs in houses, for 2nd floor slabs and balconies, and for concrete roofs.
Reinforcing should not be too close to the bottom, sides or top surface of the concrete. Usually the reinforcing is designed to have a minimum cover (distance from the outside edge of the concrete) which could be 25, 30, 40, 50 millimetres (one to two inches). The cover often depends on the structure and the conditions the concrete will be exposed to. Harsh conditions near water, and especially sea water will require a bigger distance from the edge of the concrete so that the reinforcing is better protected. Reinforcing that’s too close to the edge of the concrete could mean that the concrete can’t get around and under the steel bars so the bars could be left exposed, which means that the reinforcing will corrode and also that it won’t do its job. Water also penetrates the first layers of concrete and steel close to the surface will rust. Rusted reinforcing expands, which then breaks the concrete around the steel. This is unsightly and weakens the structure. Rusting reinforcing also leaves ugly rust marks on exposed concrete.
Unfortunately, sometimes reinforcing isn’t adequately supported and when the concrete is poured workers walk on the reinforcing squashing it deeper into the concrete and below where it should be. Reinforcing which isn’t where it should be in the concrete can result in a weakened structure and cracking on the surface. Always ensure mesh reinforcing layers are fixed correctly so that they stay in the right position, even after the concrete is poured.
For reinforcing to be effective it should be clean, not contaminated with grease and oils and relatively free of rust so that the concrete sticks to it. When reinforcing is delivered to the site it should never be stacked immediately on the ground, but should rather be raised off the ground on timber poles. Reinforcing should be used as soon as possible after it’s delivered, certainly within a few weeks.
Generally an engineer should inspect structures before concrete is poured to ensure that the correct reinforcing has been used and it’s fixed in the correct place.
Sometimes engineers replace steel reinforcing bars with fibres, which could be steel or synthetic. The fibres are mixed into the concrete during the mixing stage.
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This article is adapted from Paul Netscher's book 'An Introduction to Building and Renovating Houses; Vol 1'
Paul Netscher has written 2 easy to read books 'An Introduction to Building and Renovating Houses - Volumes 1 and 2'. An Introduction to Building and Renovating Houses Volume 1 deals with Hiring Contractors, Managing Construction and Finishing Your Home. and Designing your ideal home Volume 2 deals with Finding Your Ideal Property and Designing Your Dream Home.
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I’m a construction professional, author of several successful construction management books, and a home owner. I’ve made mistakes in construction management, I’ve seen others make mistakes, but importantly I’ve had multiple successful construction projects and I’ve learned from the mistakes. I want to share these lessons and my knowledge with you.
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